Fullerenes are a class of closed shell carbon molecules having interesting optical and opto-electronic properties. Amongst the various homologues of the series which may consist of carbon atoms ranging in number from 20 to 980 and even more (Science of Fullerene and Carbon nanotudes—M. S. Dresselhaus et al, Academic Press, 1996), the most stable and widely investigated molecules are C60 and C70 fullerenes. Structurally, C60 and C70 have similarity in the sense, they have extended network of conjugated delocalized pi-electrons which are distributed over the spherical fullerene surface. These compounds have a wide variety of remarkable properties. As in the case with many conjugated polymers, the electrical conductivity of C60 and C70 can be varied to make them insulators, semi conducting and even super conducting by controlled n-type doping. Lee W. Tutt (U.S. Pat. No. 5,172,278 dated Dec. 15, 1992) have first shown that both C60 and C70 fullerene solutions can be used as an optical limiter, transparent to low intensity light but nearly opaque above a critical intensity. One of the major difficulties in using fullerenes for all such applications is that they are unstable under ambient environmental condition. Oxygen and water in presence of light are known to react with fullerenes. So, for all practical device applications, it is necessary to encase fullerene in suitable solid host.
N. S. Sariciftci, A. J. Heeger (U.S. Pat. No. 533,183, dated 19 Jul. 1994) showed that fullerene C60 and C70 may be made technologically useful as photoconducting film by incorporating them into suitable organic conducting polymer. For optical and optoelectronic applications however, fullerene, incorporated in suitable glassy hosts, should be the most desired materials.
There are number of reports of carrying out experiment for encasing fullerene C60 and C70 in glassy hosts with limited success. In most of these cases, fullerene was tried to incorporate in silica host by sol-gel methods, e.g. methods described by B. R. Mattes, W. Duncan, J. M. Robinson, A. C. Koskelo and S. P. Love in U.S. Pat. No. 5,420,081 dated May 30, 1995 and by Sheng Dai, R. N. Compton, J. K. Young and G. Mamantov in J. of Am Cer. Soc. 75 (1992) 2865. In all these cases, preparation of thin films/plates of dimensions not larger than 13-15 mm×0.5 mm having clusters of C60 or C70 fullerene dispersed in the host, were reported. In some cases, certain derivatives of fullerene were used to prepare silica-C60 fullerene composite by sol gel method (R. Signorini et at SPIE vol. 2854, page 130). In all these above mentioned composites, a major drawback is represented by the fact that fullerene is not totally protected from the environmental degradation by the water associated with the hosts.
F. Lin, S. Mao, Z. Meng and H. Zeng, J. Qiu, Y. Yue and T. Guo in Appl. Phys. Lett., 65 (1994) 2522, described a method for preparing C60 fullerene doped phosphate glass by melting in a sealed device. The product obtained in this technique was found to contain in homogeneously dispersed fullerene clusters.
Recently R. Sahoo, S. K. Bhattacharya and R. Debnath in an Indian Patent entitled a novel process for preparing bulk monolith of Carbon sub sixty fullerene (C60)-glass-composite useful in device applications as nonlinear optical medium and optical limiter”, (No. 622/DEL/2001 dated May 29, 2001) described a method for preparing bulk monolith of fullerene (C60)-glass of different compositions.
According to the above Patent Application, the glass composition contains 0-50 mole % phosphorous pentaoxide (P2O5), 0-40 mole % boron oxide (B2O3), 0-30 mole % bismuth oxide (Bi2O3), 0-1 mole % silicon dioxide (SiO2), 0-1 mole % aluminum oxide (Al2O3), 10-45.5 mole % zinc oxide (ZnO2), 10-40 mole % lead oxide (PbO), 0-0.1 mole % tin dioxide (SnO2), 0-5 mole % M2O where M=Na, K, Li and 0.05-0.10 wt % fullerene (C60).
However, it was noticed that the glass compositions used in this process have limitations in dissolving fullerene (C60) in a high concentration. The composition described in the aforesaid Indian Patent Application is capable of keeping fullerene in the medium only in the dispersed condition. As a result, relatively higher concentration of fullerene namely fullerene concentration greater than 0.10 wt % cannot be incorporated in the glass composition. As it is preferable to incorporate higher and higher amounts of fullerene in the glass composition for device application such as optical limiter and non-linear media, in the present invention, the aforesaid patent application also has some drawbacks.
It is a commonly known principle that effective dissolution of a dopant in a host is achieved only when there occurs some short of interaction between the dopant and the host. The compositions disclosed in the patents in most cases, are not interactive with fullerene (C60), although in some cases they are weakly interactive. Hence the processes can prepare glasses only with low concentration of fullerene.
The main drawbacks of the existing methods of preparation of fullerene doped glasses are as follows:    1) In the case of the fullerene doped sol-gel silica glass, fullerenes are not totally protected from environmental degradation as well as from the inherent water molecules associated with the host.    2) The sealed device melting method of preparing fullerene-phosphate glass composite does not yield a homogeneous product and at the same time suffers from its limitation in offering samples of useful dimensions.    3) Fullerene (C60)-glass having high concentration of dissolved fullerene (C60) can not be achieved using the glass compositions disclosed so far by the earlier workers. Even, the compositions recently suggested in the Applicant's earlier Indian Patent Application No. 622/DEL/2001 dated May 29, 2001 have limitations in dissolving fullerene (C60) in a high concentration. This is because, compositions suggested in these earlier studies are in most cases, either not at all interactive or weakly interactive with fullerene (C60).
In order to overcome all the above drawbacks, it is essential to provide a novel glass composition which is not only capable of dissolving high concentration of fullerene, but also capable of protecting the fullerene from environment degradation. Also, care should be taken to avoid formation of clusters of fullerene during the incorporation.
Keeping in mind all the above-criteria, the Inventors have devised a novel composition which is capable of incorporating 0.15 to 0.20 wt % of fullerene in the glass. The synergistic glass composition of the present invention is formulated judging the chemical reactivity of the ingredients as well as the reactivity of the resulting glass with the incorporated fullerene (C60) to form bonds so that the glass can dissolve high concentration of fullerene (C60) in its matrix. The Inventors have noticed that the ingredients and their concentration play a very vital role in overall properties of the glass thus produced. If the ingredients are changed or if the concentrations of the ingredients are varied beyond the ranges specified in the invention, the properties of the glass thus obtained are not satisfactory.